Zener diode when used as a voltage regulator is connected
a) in forward bias
b) in reverse bias
c) in parallel to the load
d) in series with the load 
 

•  A reverse voltage is applied across the diode and reaches the breakdown voltage of the device, a significant current begins to flow through the diode. 
• As a result, a huge current will flow through it at a constant voltage. 
• The load was therefore constantly connected to it in parallel.

Hence the correct answer is b and c are correct.